The economic viability of well drilling operations to recover underground gas can be heavily affected by the efficiency with which the drilling procedure is conducted. In a common method of producing gas from a gas well, the well is drilled and a relatively large casing sleeve inserted into the bore to a depth less than the depth from which gas is to be produced. The bore is then continued to the depth of gas production using a smaller diameter drill. A length of smaller diameter casing having a check valve at its lower end to prevent upward flow is forced down into the well, sealed to the larger casing to extend beyond the end of the larger casing to and beyond the producing area. This smaller diameter casing may be several hundred feet in length and comprise several conventional lengths of pipe, each prepared prior to use in the well by milling a plurality of circumferentially spaced holes at axially spaced intervals. These holes are tapped and initially sealed with aluminum cup-shaped plugs which have their rims screwed into the tapped holes with their open cup-rim ends flush with the exterior casing wall and their closed cup-ends protruding beyond the interior casing wall. Being sealed in this manner, the casing may be inserted into the well and through pressurized areas without allowing any significant escape or conduction of gas. Once the prepared well casing is in place within the well, the cup-end protrusions at the interior of the smaller casing are milled out using a cylindrical milling bit which shears off the aluminum plugs leaving the holes open for the passage of gas. Typically, the drill string and milling unit are removed from the well and a "production string" is reinserted to conduct gas to the ground surface.